In this study we investigated the effect of a mechanical perturbation of unexpected timing during human running. With the use of a powered exoskeleton, we evoked a dorsiflexion of the right ankle during its swing phase while subjects ran on a treadmill. The perturbation resulted in an increase of the right ankle dorsiflexion of at least 5°. The first two as well as the next five steps after the perturbation were analyzed to observe the possible immediate and late biomechanical adjustments. In all cases subjects continued to run after the perturbation. The immediate adjustments were the greatest and the most frequent when the delay between the right ankle perturbation and the subsequent right foot touch-down was the shortest. For example, the vertical impact peak force was strongly modified on the first step after the perturbations and this adjustment was correlated to a right ankle angle still clearly modified at touch-down. Some late adjustments were observed in the subsequent steps predominantly occurring during left steps. Subjects maintained the step length and the step period as constant as possible by adjusting other step parameters in order to avoid stumbling and continue running at the speed imposed by the treadmill. To our knowledge, our experiments are the first to investigate perturbations of unexpected timing during human running. The results show that humans have a time-dependent, adapted strategy to maintain their running pattern.